The present invention is directed to the art of cathodic protection systems. It finds particular application in conjunction with reference electrodes used to monitor cathodic protection systems and corrosion and will be described with particular reference thereto. However, it is to be appreciated that the invention has broader application and may be advantageously employed in other environments.
Reference electrodes are used in cathodic protection work to evaluate the effectiveness of cathodic protection systems. They are also used with freely corroding structures to determine where the corrosion activity is the greatest. A reference electrode is commonly made from a pure metal immersed in a saturated solution of its own metal salt. In so doing, the energy level of that metal is stabilized. If the metal, in its saturated solution of its own salt, is placed in contact with a common electrolyte in which a metal component is either immersed or buried, one can measure the energy imbalance and know that all of the energy imbalance or change with time or from any known value is attributable to what is happening on the other structure and not with the reference electrode.
A copper/copper sulfate reference electrode is commonly used. A pure copper rod is immersed into a saturated solution of copper sulfate. The copper rod and saturated solution are placed inside of a plastic tube that has a porous plug tip in the end of it which permits the ions within the tube to come in contact with the ions in soil or water. The copper rod is contacted with a test lead of a voltmeter, and the other test lead of the voltmeter is connected to the structure to be tested. The porous plug tip is placed in contact with the earth and the energy imbalance is measured.
For example, if a measurement is made as just described on an underground steel pipe line, the structure will be in a freely corroding condition at an energy level of about half a volt. That would be -0.5 volts with respect to the copper/copper sulfate reference electrode. If the steel pipe line is brought under cathodic protection, that energy level will be raised to a value of about -0.85 volts. This is one of the criteria used for corrosion effectiveness. That is, an energy level shift to -0.85 volts will free the structure from all further corrosion. However, the readings are inconsistent. Different readings are produced based on where the reference electrode is placed. Hence, reference electrodes are often placed at permanent sites, most commonly down near the structure surface underground.
There are some problems associated with placing reference electrodes in the earth such as near the steel structure in question. For one, liquid electrolyte inside the cell will tend to permeate out into the surrounding environment with time. As a result, the cell becomes depleted, or, conversely contaminants are infiltrated into the cell so that there is no longer a pure copper rod in a saturated solution of only the pure metal salt. Impurities can react with a copper surface and contaminate the cell and cause its energy level to change. Various approaches are used to minimize that effect from happening, but none of them work very well.
Also, if the reference electrode is buried in the ground, it will not be known when the actual energy level has started to change on the reference electrode. Moreover, the presence of the electrode so close to the structure being monitored can interfere with cathodic protection to the device. If the solution leaks out, it can increase the corrosion of the steel.
Little has been done to improve upon the negative aspects of electrodes used for monitoring cathodic protection or corrosion. One attempt, however, has been to run a capillary tube or luggin probe (i.e., a small tube or capillary filled with electrolyte that terminates near to the metal surface under study and used for the purpose of providing an ionically conducting path without diffusion between an electrode under study and a reference electrode) down from the surface of the earth to the location right next to a permanently installed reference electrode. That capillary tube is filled with a liquid electrolyte. A reading can be made through that capillary tube moisture path. In such a system, a reference cell is placed at the top of the tube and contacted with the moisture path provided by the capillary tube. A reading can be made down through the plastic tube to the reference cell down in contact with the earth to measure the energy imbalance between the tube and cell. The reason for this apparatus is that if there were a cell up at the top of the earth and another one down in the earth about four or five feet away, there could be a natural voltage gradient in the earth that would distort the reading. Without the capillary tube, it is virtually impossible to determine an error in the reading. By using the tube, the reading is actually made between the tip of the tube down in the earth and the cell that was buried there.
There are a number of problems with this structure that employs a capillary tube or luggin probe. Liquid electrolyte poured down the tube evaporates or leaks out over time. It is necessary to keep replenishing it. This is a cumbersome process that many would prefer to avoid. Also, the permanent cell placed down in the earth is an expensive item. It may cost around $50 to $150 to manufacture.
In order to overcome the cumbersome and uneconomical aspects associated with using the capillary tube, it is desirable to develop an improved capillary tube that will reduce the possibility of electrolyte leakage therefrom. Further, it is desirable to develop a capillary tube that can remain in place for an extended period of time without replacement.
The present invention contemplates a new and improved reference electrode capillary tube which overcomes the above-referenced problems and others.